A high-frequency communication filter can be constituted by combining a plurality of resonators that use surface acoustic waves (SAW) or bulk acoustic waves (BAW) generated by applying an AC voltage to a piezoelectric material. Such filters are characterized by allowing only electrical signals in a specific frequency band to pass. Such filters are installed in a mobile communication (high-frequency wireless communication) device such as a cellular telephone terminal, a personal handyphone system (PHS) terminal or a wireless local area network (LAN) system, for example. Japanese Laid-Open Patent Publication No. 2001-24476 discloses a ladder filter having a similar function to a SAW filter or the like and constituted with film bulk acoustic resonators (FBARs) connected in a ladder configuration.
In recent years, a boundary wave filter using boundary waves that mainly propagate along the boundary between a piezoelectric substrate and a medium formed on the piezoelectric substrate has also been developed. Such filters using SAW, BAW or boundary waves have a small profile compared with other dielectric filters or ceramic filters and have a steep roll-off. Therefore, such filters are suited to mobile communication components such as cellular telephones that are compact and require a narrow relative bandwidth.
A duplexer is an application component of such a filter. A duplexer has transmit/receive functions, and is, for example, used in a wireless apparatus with different frequencies for transmission signals and reception signals.